Actually, you don't need to do anything except wait for it to go away, as hard as this may be! This type of algae bloom is typical for a newly filled pond and it should clear on its own in a few weeks if you have included a good mix of plants in your pond. You should have surface covering plants such as water lilies to cover about three quarters of the surface, under water oxygenating plants in a generous quantity (this is the group many people skimp on) and finally some bog plants or marginal plants. All of these work together to shade the surface and keep sunlight from penetrating into the water and also to use up the nutrients in the water -- nutrients are contained in plant soil and fertilizers, in fish excrement, and sometimes in the water itself. Excess fish food will also contribute nutrients, so be careful not to overfeed your fish. By all means do not empty and refill the pond as this will result in the same thing happenening again. Try to be patient, allow time for the plants to grow and use the nutrients, and give the pond a chance to "come into balance". You may find that the pond does this each spring for a short period while the water temperature rises and the plants take a little time to catch up with and outcompete the algae in the water.